In digital communications circuitry, higher data rates require faster signal edge transitions, i.e., rise and fall times, and shorter bit periods. The potential for undesirable crosstalk between adjacent channels increases with increased edge speed, and shorter bit periods reduce tolerance to the adverse effects of crosstalk. Under such conditions, minimizing crosstalk is important to ensuring proper data reception.
Optical fiber-based communication systems commonly include a receiver device that has one or more photodiodes or other opto-electronic detectors. The receiver device converts received optical signals into electrical signals. The outputs of the receiver device, e.g., a first integrated circuit chip, are connected to the inputs of another device, e.g., a second chip, which processes the received electrical signals.
As illustrated in FIGS. 1 and 2, for example, an opto-electronic receiver chip 10 and a processing chip 12 may be integrated within a multi-chip module 14. A plurality of optical fibers 16, 18, 20, etc. (with additional fibers and other elements that are not shown for purposes of clarity being indicated by an ellipsis (“ . . . ”) symbol) are coupled to module 14 via optical fiber connectors 22, 24, 26, etc. Opto-electronic receiver chip 10 includes a corresponding plurality of opto-electronic detectors (e.g., photodiodes) 28, 30, 32, etc. The optical signal, i.e., light, emanating from the end of each fiber 16, 18, 20, etc., is received by a corresponding detector 28, 30, 32, etc., which, in response, outputs an electrical signal representative of the received optical signal.
As described in further detail below, opto-electronic receiver chip 10 and a processing chip 12 can be electrically coupled to one another by wirebonds. As well understood in the art, wirebonding is a solid-phase welding process by which a very fine gauge uninsulated wire is welded to the metallic material of a corresponding wire bond pad on a chip, leadframe or other structure. A wire and bond pad are placed in contact with each other and welded together using heat, pressure or some combination thereof, typically through thermocompressive, ultrasonic or thermosonic processes. The term “wirebond” refers to both the welding process and the resulting connection formed in this manner.
A first group of two wirebonds 34 and 36 couples opto-electronic detector 28 to processing chip 12; a second group of two wirebonds 38 and 40 couples opto-electronic detector 30 to processing chip 12; an Nth group of two wirebonds 42 and 44 couples opto-electronic detector 32 to processing chip 12; etc. For purposes of convenience of description herein, each such group is described as corresponding to an optical “channel.” The system can have any number (N) of such channels.
The N-terminal of, for example, opto-electronic detector 28 is coupled via wirebond 34 to a positive power supply voltage (V_SUPPLY) provided by power supply circuitry. The power supply circuitry can include filter circuitry, represented by the combination of a resistor 46 coupled to V_SUPPLY and a capacitor 48 coupled to ground. Likewise, the N-terminal of opto-electronic detector 30 is coupled via wirebond 38 to power supply circuitry represented by a similar combination of a resistor 50 and a capacitor 52, the N-terminal of opto-electronic detector 32 is coupled via wirebond 42 to power supply circuitry represented by a similar combination of a resistor 54 and a capacitor 56, etc. The P-terminal of opto-electronic detector 28 is coupled via wirebond 36 to the input of an amplifier 58; the P-terminal of opto-electronic detector 30 is coupled via wirebond 40 to the input of an amplifier 60; the P-terminal of opto-electronic detector 32 is coupled via wirebond 44 to the input of an amplifier 62; etc. In FIGS. 1 and 2, the N-terminal signal path and P-terminal signal path of the optical channels (which can be designated channel “1” through channel “N” for purposes of convenience) are labeled “CH1_N” and “CH1_P”, respectively, through “CHN_N” and “CHN_P”, respectively. Although not shown for purposes of clarity, the outputs of amplifiers 58, 60, 62, etc., are coupled to other circuitry in processing chip 12 involved in the processing of the received signal, as is well understood in the art.
In operation, each of opto-electronic detectors 28, 30, 32, etc., generates an electric current signal in response to the optical signal it receives. The above-described power supply circuitry supplies the N-terminal of each of opto-electronic detectors 28, 30, 32, etc., with a voltage. Due to the filtering or damping effect of the above-described resistances and capacitances, this voltage is relatively constant. The current, however, changes in response to the optical signal. Each of amplifiers 58, 60, 62, etc., generates an output signal in response to the current.
A changing voltage in a group of one or more of the above-described wirebonds can give rise to capacitively coupled crosstalk in an adjacent group of one or more wirebonds. Similarly, magnetic flux generated by a changing current in a group of wirebonds can give rise to inductively coupled crosstalk in an adjacent group of wirebonds. It is desirable to minimize such crosstalk.